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In Situ Optical Measurements of Chang'e-3 Landing Site in Mare Imbrium: 1. Mineral Abundances Inferred from Spectral Reflect

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In Situ Optical Measurements of Chang'e-3 Landing Site in Mare Imbrium: 1. Mineral Abundances Inferred from Spectral Reflect PUBLICATIONS Geophysical Research Letters RESEARCH LETTER In situ optical measurements of Chang’E-3 landing 10.1002/2015GL065273 site in Mare Imbrium: 1. Mineral abundances Key Points: inferred from spectral reflectance • We obtained in situ visible-NIR reflectance spectra of the lunar Hao Zhang1, Yazhou Yang1, Ye Yuan1, Weidong Jin1, Paul G. Lucey2, Meng-Hua Zhu3, ’ surface at Chang E-3 landing site 4 4 5 5 5 1 • These in situ spectra have less mature Vadim G. Kaydash , Yuriy G. Shkuratov , Kaichang Di , Wenhui Wan , Bin Xu , Long Xiao , 1 6 features than that measured remotely Ziwei Wang , and Bin Xue • Using the spectral lookup table, we identified large amount of olivines 1Planetary Science Institute, School of Earth Sciences, China University of Geosciences, Wuhan, China, 2Hawaii Institute of Geophysics and Planetology, University of Hawaii, Honolulu, Hawaii, USA, 3Space Science Institute, Macau University of 4 Supporting Information: Science and Technology, Taipa, China, Astronomical Institute, Kharkov V.N. Karazin National University, Kharkov, Ukraine, 5 • Texts S1 and S2, Figures S1–S5, and State Key Laboratory of Remote Sensing Science, Institute of Remote Sensing and Digital Earth, Chinese Academy of Tables S1–S4 Sciences, Beijing, China, 6Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi’an, China Correspondence to: H. Zhang, Abstract The visible and near-infrared imaging spectrometer on board the Yutu Rover of Chinese Chang’E-3 [email protected] mission measured the lunar surface reflectance at a close distance (~1 m) and collected four spectra at four different sites. These in situ lunar spectra have revealed less mature features than that measured remotely by Citation: spaceborne sensors such as the Moon Mineralogy Mapper instrument on board the Chandrayaan-1 mission Zhang, H., et al. (2015), In situ optical and the Spectral Profiler on board the Kaguya over the same region. Mineral composition analysis using a measurements of Chang’E-3 landing site in Mare Imbrium: 1. Mineral abundances spectral lookup table populated with a radiative transfer mixing model has shown that the regolith at the inferred from spectral reflectance, landing site contains high abundance of olivine. The mineral abundance results are consistent with that inferred Geophys. Res. Lett., 42, 6945–6950, from the compound measurement made by the on board alpha-particle X-ray spectrometer. doi:10.1002/2015GL065273. Received 8 JUL 2015 Accepted 12 AUG 2015 1. Introduction Accepted article online 18 AUG 2015 Published online 10 SEP 2015 The chemical, mineral, and physical properties of the lunar surface carry information regarding the geological history and space environment of the Moon [e.g., Lucey et al., 2006]. Currently, it is believed that the abundance and spatial distributions of the dominant minerals of mare basalts including olivine (OLV), clinopyroxene (CPX), orthopyroxene (OPX), plagioclase (PLG), and ilmenite are determined by a global fractionation event of magma ocean induced flotation of anorthosite-rich crust and the subsequent magmatism of basalts derived from melting of the mantle [Staid and Pieters, 2001]. Thus, the accurate mapping and validations of mineral types, distributions, and abundance play a key role in understanding the evolutionary history of the Moon. Visible and near-infrared spectroscopy is a powerful tool in remote characterizations of the physical and mineralogical properties of the Moon [e.g., Shkuratov et al., 2011]. However, there is a large gap in spatial scale between spectral measurements from orbit with resolutions at best of tens to hundreds of meters and the spectral properties of individual samples returned from the Moon and analyzed in terrestrial laboratories [Ohtake et al.,2013;Pieters et al., 2013; Donaldson Hanna et al., 2014]. The successful landing of the Chang’E-3 (CE3) lander and the deployment of the Yutu Rover at Mare Imbrium [Xiao et al., 2015] has provided the first in situ spectral analysis of the lunar surface. The Visible and Near-infrared Spectrometer (VNIS) on board the rover captured the spectral features of the lunar surface at centimeter to meter scales that can bridge this gap in scales to help us better understand the lunar environment and evolutionary history. 2. Instrument, Measurement, and Data Reductions The VNIS and the Active Particle X-ray Spectrometer (APXS) instruments are located on the lower left and lower right on the front surface of the rover, respectively. The VNIS is located about 0.7 m above the ground and looks down at the lunar surface at a nominal 45° angle and roughly images an area of 13 by 13 cm (Figure S1 in the supporting information). The APXS detector is situated at a robotic arm and looks at the lunar surface at a distance of 20 mm. The VNIS consists of a Complementary Metal-Oxide Semiconductor (CMOS) imager with ©2015. American Geophysical Union. 256 by 256 pixels and a short-wavelength near-infrared (SWIR) detector with one-single pixel [Liu et al., 2014] All Rights Reserved. using acousto-optic tunable filters for wavelength discrimination. The CMOS camera spans the wavelength ZHANG ET AL. IN SITU LUNAR REFLECTANCE SPECTROSCOPY 6945 Geophysical Research Letters 10.1002/2015GL065273 Figure 1. Orthorectified image of the landing area taken by the Landing Camera. The landing site is marked by the yellow dot (Lander), and the rover’s path with locations of the four VNIS measurements is also marked. The inset is a Lunar Reconnaissance Orbiter Camera image taken on 17 February 2014 showing the increased surface albedo after landing. range from 450 to 945 nm with a spectral resolution of 2–7 nm, and the SWIR detector works from 900 to 2395 nm with a spectral resolution of 3–12 nm, respectively, and both operate at 5 nm spectral sampling. The single-pixel SWIR detector images a circled area within the field of view of the CMOS camera (Figure S1). The typical signal-to-noise ratios of the CMOS and SWIR sensors are better than 30 dB (1000:1) when making measurements in the typical lunar environment (Report of Scientific Validations of Visible-Near-infrared Imaging Spectrometer of the Chang’E 3 Mission, Document # CE3-GRAS-CSSY-003-F3, version 1.0, Released 16 May 2012). As indicated in Figure 1, the Yutu rover path is roughly a flipped L shape. The VNIS collected individual spectra at four different sites (Nodes E, S3, N203, and N205), from 23 December 2013 to 14 January 2014 (see Table S1 À À À for measurement locations and times). The calibrated radiance values (μWcm 2Sr 1 nm 1) were provided by the payload team after performing dark current subtractions and several multiplicative corrections including flat field, geometric corrections, and radiometric calibration. Reflectance expressed in terms of the reflectance factor [Hapke, 2012] was obtained by using the solar irradiance calibration method (see supporting informa- tion (SI)). In addition, two APXS measurements were also made at Node S3 and Node N205 and the results are summarized in Table 1. Figure 2 displays the VNIS reflectance spectra with photometric corrections which bring the reflectance to the NASA Reflectance Experiment Laboratory configuration (i = 30°, e = 0°) (SI). To compare the in situ data with remote measurements made over the same region, we also plot 25 reflectance spectra measured by the Chandrayaan-1 Moon Mineralogy Mapper (M3)[Pieters et al., 2009] (http://pds-imaging.jpl.nasa.gov/data/m3/ CH1M3_0004/DOCUMENT/DPSIS.PDF. It should be noted that the M3 level 2 data product released is actually the radiance factor which should be divided by the cosine of the incident zenith to obtain the reflectance defined in equation (S2)). These 25 pixels form a 5 by 5 “matrix” with its central element (element [3, 3]) being centered on the location of the CE3 lander. Since each M3 pixel has a spatial resolution of 140 m, the total area ZHANG ET AL. IN SITU LUNAR REFLECTANCE SPECTROSCOPY 6946 Geophysical Research Letters 10.1002/2015GL065273 Table 1. Elemental Abundance Results From the Two APXS Measurements covered by these 25 pixels is roughly a Made at Node S3 and Node N205 square of 700 by 700 m. In Figure 2, Node S3 Node N205 we also plot Kaguya’s Spectral Profiler Components Wt (%) Fit Err (%) Wt (%) Fit Err (%) (SP) measurements of a nearby site with center latitude 44.162°N and center MgO 10.503 7.487 10.116 6.037 Al O 10.311 2.325 10.725 2.267 longitude 340.514°E. The level 3 SP 2 3 fl SiO2 41.882 2.666 40.456 2.934 product is the bidirectional re ectance K2O 0.131 8.477 0.144 9.260 [Yamamoto et al., 2011]; and thus, it is CaO 10.753 0.336 11.125 0.274 multiplied by π/cos(30°) to be converted TiO2 4.173 0.395 4.395 0.244 to the reflectance factor [Hapke, 2012]. FeO 21.247 1.037 22.038 0.863 It is seen that all CE3 spectra have deeper 1 μm absorption band, and even the darkest CE3 spectrum, Node N205, is nearly 3 times brighter than the average M3 spectrum. There are several possible reasons. First, during the landing process, the descent engines should blow away the topmost regolith layers, destruct the intricate “fairy castle” structure of natural soils [Pieters et al., 2013], and expose grains underneath the most weathered surface. Therefore, the VNIS may have measured less mature and less weath- ered fresh ejecta regolith. Second, as shown by the Lunar Reconnaissance Orbiter Camera (the inset of Figure 1), CE3 was landed near a 450 m diameter fresh crater and the ejecta covers all the path of CE3 rover [Xiao et al., 2015]. Additionally, the CE3 used our in situ phase curve [Jin et al., 2015; W.
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